Gasification of carbonaceous materials to produce producer gas or synthesis gas or syngas comprising carbon monoxide and hydrogen is well known in the art. Typically, such a gasification process involves a partial oxidation or starved-air oxidation of carbonaceous material in which a sub-stoichiometric amount of oxygen is supplied to the gasification process to promote production of carbon monoxide as described in WO 2009/154788. As described in WO 2009/154788, a gasification process can further be influenced by addition of one or more of steam and carbon dioxide (CO2). Success of a gasification process greatly depends on quality of syngas produced. Increased content of carbon monoxide (CO) and hydrogen (H2) is desirable in syngas produced. In other words, contents of diluents such as carbon dioxide (CO2), nitrogen (N2) should be as low as possible especially for use of product syngas for heating value or for producing chemicals.
Various mineral matters often form part of carbonaceous materials. While the hydro-carbonaceous part of carbonaceous materials converts to CO, CO2 and H2, the mineral matters get separated from the hydro-carbonaceous part and together with any unconverted carbonaceous material or unconverted carbon form ash. The amount and composition of ash (e.g. carbon content) can have an impact on the smooth running of the gasifier as well as on the disposal of ash. Melting and agglomeration of ash in the gasifier may cause slagging and clinker formation that can lead to partial or complete blocking of gasifier. It is, therefore, advantageous to have a gasification process that avoids the melting of ash. It is also advantageous to have a low content of unburned fuel or carbon in ash.
James T. Cobb, Jr. (“Production of Synthesis Gas by Biomass Gasification,” James T. Cobb, Jr., Proceedings of the 2007 Spring National AIChE Meeting, Houston, Tex., Apr. 22-26, 2007) describes a Consutech Gasifier (BRI Energy LLC), the first stage of which is a standard step-grate combustor (frequently used as an MSW incinerator) that operates as a gasifier at 950° F. using oxygen-enriched air. The second stage is a heat treater that operates at 2000-2250° F. and uses minimal oxygen to crack tars.
WO 2009/154788 describes a two stage gasifier in which carbonaceous material is fed to the first stage in which air, oxygen-enriched air or pure oxygen can be injected at a controlled rate. The first stage temperature and oxygen input is controlled such that only partial oxidation of carbonaceous material occurs. The gaseous product from the first stage moves to the second stage. Ash is removed from the first stage. Pure oxygen is introduced into the second stage in order to accomplish cracking and partial oxidation of any tar contained in the gaseous stream from the first stage.
A two stage gasifier such as that described in WO 2009/154788 can be effective in producing syngas from various waste carbonaceous materials and good quality syngas can be produced, however, a high carbon content is generally observed in ash produced from this gasification process.